The present invention relates to fastening composite material parts that are used in the after-bodies of aeroengines such as turbojets. In order to reduce after-body weight, it is known to make one or more after-body parts, such as the exhaust cone, the primary nozzle, and/or the mixer, out of a ceramic matrix composite (CMC) material rather than out of a metallic material. Such parts present a coefficient of thermal expansion that is small compared with that of the metal casings of the engine to which they need to be mounted. In order to compensate for differential expansion between these elements, the CMC part is mounted on the casing made of metallic material by means of resiliently flexible fastener tabs that are generally made of a refractory metallic material.
The use of flexible fastener tabs for mounting a CMC mixer is described in particular in the following documents: US 2008/115484; WO 2008/139114; and FR 2 912 469. Document WO 2008/148999 describes using flexible fastener tabs for fastening exhaust nozzles in a gas turbine. Document EP 1 873 385 discloses and after-body assembly comprising an exhaust cone held facing an annular part by means of flexible fastener tabs.
In addition to compensating differential expansion, the fastener tabs must also enable the assembly to withstand the normal and limit loads that are encountered by the engine, i.e. they must ensure that the parts are held relative to one another under such loads. The “loads” (or load factors) correspond to the forces that are associated with the accelerations to which the engine is subjected, and they are expressed as a number of gs. Limit loads correspond to the greatest loads that might be encountered under operating conditions (air pocket or very difficult landing). Nevertheless, safety regulations in the field of aviation also define an even greater level of loading referred to as the “ultimate” load or the “extreme” load. In the absence of any particular provisions, the extreme load is generally defined by multiplying the limit load by a safety factor. For example, the ultimate load threshold may be 1.5 times the limit load threshold.
It is difficult to justify making the flexible fastener tabs to be capable of withstanding ultimate loads. In order to withstand ultimate loads, the flexible fastener tabs would need to present very considerable thickness both in the tab body forming the resiliently flexible link between the two parts and in the portions where the tab is fastened to the CMC part. Such reinforcement of the fastener tabs would lead to an excessive increase in overall weight, in contradiction to the weight saving to be expected from using CMC parts, and also making the connection stiffer.